Ceramic material

ABSTRACT

A ceramic material which comprises a composite of zirconia and O&#39;-sialon or silicon oxynitride. 
     The ceramic material may comprise a dispersion of zirconia in an O&#39;-sialon matrix and such a dispersion is obtained when the amount of zirconia is from 5 to 30 volume percent, preferably 15 to 25 volume percent, based on the total volume of the composition.

The present invention relates to an improved ceramic material and, inparticular, to an improved engineering ceramic material.

Engineering ceramics are materials such as the oxides,nitrides andcarbides of the metals silicon, aluminium, boron and zirconium. They arecharacterized by great strength and hardness; properties which in theorycan be retained to very high (>1000° C.) temperatures. Two of the mostpromising types of ceramic are the sialon family, and the zirconiafamily.

The sialons are based on the elements Si, Al, O, N, hence the acronym. Asuccessful commercial sialon is the β'-sialon which has the β-Si₃ N₄crystal structure, but with some of the silicon atoms replaced byaluminium atoms, and for valency balance some nitrogen atoms replaced byoxygen atoms. The sialons are usually formed by mixing Si₃ N₄, Al₂ O₃,ALN with a metal oxide (often Y₂ O₃), compacting the powder to thedesired shape, and then firing the component at 1750° C. for a fewhours. The function of the metal oxide is to react with the alumina andthe silica layer (which is always present on the surface of each siliconnitride particle), to form a liquid phase which dissolves the reactantsand precipitates the product. The liquid phase (which still containsdissolved nitrides), cools to form a glass between the β'-sialon grains.Typically, an Y₂ O₃ densified O'-sialon contains about 15 volume precentof Y-Si-Al-O-N glass and 85 volume percent β'-sialon. At temperaturesabove 800° C. this glass begins to soften and the strength decreases.The glass/sialon can be heat treated at ˜1300° C. to crystallise theglass. In the case of β'-sialon and glass, the glass crystallises togive Y₃ Al₅ O₁₂ (yttro garnet or YAG) and a small amount of additionalβ'-sialon. With glass/O'-sialon the crystallisation produces Y₂ Si₂ O₇(yttrium disilicate) plus a small amount of additional O'-sialon. Thiscrystallisation process reduces the room temperature strength of thematerial, but this reduced strength is maintained to higher temperature.The reason that crystallisation reduces strength is not completelyunderstood, but is probably becasue the crystalline YAG occupies asmaller volume than the glass it replaces; crystallisation leaves smallcracks. The grain boundary phase is a necessary evil in these materials,it is a remnant of the densification process.

Another promising ceramic family is based on tetragonal zirconia, ZrO₂.The tetragonal zirconia is dispersed in a matrix typically mullite,alumina or cubic zirconia. The tetragonal zirconia toughens by a processknown as transformation toughening. Basically, the composite is fired athigh temperature (at least 1100° C.), when the ceramic densifies, andthe zirconia is in its high temperature tetragonal form. On cooling, thetetragonal zirconia attempts (and fails) to transform to its lowtemperature monoclinic form. The matrix constrains the zirconia in itstetragonal form which at room temperature is metastable. Thistransformation would be accompanied by a 3-5 volume percent increase ineach zirconia crystal. The effect is to put the entire matrix intocompressive stress, rather like prestressed concrete. Any crack runninginto such a ceramic tends to trigger the tetragonal to monoclinictransformation which generates compressive stresses which tend to closeoff the crack. The process becomes more efficient, the stiffer thematrix, because the stiff matrix is better able to constrain themetastable tetragonal form at room temperature. The process is lesseffective at high temperature, and there is no toughening at all above900° C. because the tetragonal zirconia is now stable not metastable.

Whilst it would be desirable to attempt to zirconia toughen sialonsbecause they are stiff (and hard and strong) but are also quite tough tostart with, workers in this field have found that zirconia reactschemically with β'-sialon and is partly reduced to zirconiumoxynitrides.

We have now surprisingly found that O'-sialon does not react withzirconia but instead forms a stable composite with it. The presentinvention is based upon this discovery.

Accordingly, the present invention provides a ceramic matarial whichcomprises a composite of zirconia and O'-sialon or silicon oxynitride.

The ceramic material of the invention may contain from 5 to 95 volumepercent zirconia based on the total volume of the composition.

The ceramic material may comprise a dispersion of zirconia in anO'-sialon matrix and such a dispersion is obtained when the amount ofzirconia is from 5 to 30 volume percent, preferably from 15 to 25 volumepercent based on the total volume of the composition.

The ceramic materials of the present invention may include in theO'-sialon matrix a solid solution of zirconia with yttria, ceria,lanthanum oxide, calcium oxide, magnesium oxide or a rare earth metaloxide.

The present invention furthermore provides a process for the preparationof a ceramic material as hereinbefore described which process comprisesthe reaction sintering at a temperature in the range of from 1500° to1750° C. of zircon, silicon nitride and optionally alumina or aprecursor for alumina, optionally in the presence of a reactionsintering aid or a precursor therefor.

The primary function of the metal oxide sintering aid is to form a solidsolution with the zirconia. Thus, the sintering aid reacts initiallywith the alumina and the surface layer of silica on the silicon nitrideto form a transient liquid phase which dissolves the silicon nitride andthe zircon and from which the zirconia and the O'-sialon precipitate.

The sintering aid used in this process may be, for example, yttria,ceria, lanthanum oxide, calcium oxide, magnesium oxide of a rare earthmetal oxide, or a precursor for one of these compounds. Thus, we havefound that, the alumina for the above described process and thesintering aid may be provided by the use of a spinel.

Preferred spinels for use in the process of the invention are those ofmagnesium, calcium or barium, with the compound of the formula MgAl₂ O₄being particularly preferred for use.

The spinel is incorporated into the mixture which is sintered in anamount sufficient to provide the desired amount of aluminium in thefinal O'-sialon matrix. The spinel is thus preferably used in an amountof up to 10% by weight based on the weight of the zircon and siliconnitride, preferably in an amount of from 6 to 8% by weight.

Other precursors of various of the components incorporated into themixture reaction sintered according to the above process may also beused. Thus, the ceramic material of the invention comprising adispersion of zirconia in an O'-sialon matrix may be prepared byreaction sintering a mixture of zircon, silicon nitride, a metalsilicate and alumina.

The metal silicate may be, for example, a silicate of calcium, magnesiumor barium. It will be appreciated that on heating to sinteringtemperatures the metal silicate will react with some of the zircon andsilicon nitride to form a liquid phase which promotes reaction anddensification by a solution-precipitation mechanism. The oxides whichmay be used as sintering aids may also be provided by precursors such ascarbonates or bicarbonates which decompose to the oxide under thesintering conditions. For example calcium oxide and magnesium oxide assintering aids may be provided by calcium carbonate or magnesiumcarbonate respectively.

We have also found that instead of using zircon (ZrSiO₄) in the processas described above, a mixture of zirconia (ZrO₂) and silica (SiO₂) maybe used. This modification has the advantage that, whereas in zircon theratio of ZrO₂ to SiO₂ is fixed, it is possible to vary the ratio ofzirconia to silica, as required. This may, in some instances, beparticularly advantageous.

The present invention thus provides in a further aspect a process forthe preparation of a ceramic material comprising a composite of zirconiaand O'-sialon, which process comprises the reaction sintering at atemperature in the range of from 1500° to 1750° C. of a mixture ofzirconia, silica, silicon nitride and optionally alumina or a precursortherefor, optionally in the presence of a reaction sintering aid or aprecursor therefor.

The reaction sintering aid, or the precursor therefor, used in thisalternative embodiment of the invention is as hereinbefore described.Furthermore, the alumina for this process and the sintering aid may beprovided by the use of a compound, e.g. a spinel as hereinbeforedescribed.

The present invention will be further described with reference to thefollowing Examples.

EXAMPLE 1

The following compositions were ball milled for 24 hours underisopropanol, using a 3mm zirconia mixing media. The slurry was pan driedand the powder isostatically pressed at 20,000 psi into billets.

The ratio of silicon nitride to zircon to alumina was kept constant,whilst the yttria content was varied from 0.8% (by weight) to 20% (byweight).

Composition A

    ______________________________________                                        Zircon          56.0 g                                                        Silicon nitride 38.5 g                                                        Alumina          4.7 g                                                        Yttria           0.8 g                                                        ______________________________________                                    

Composition B

    ______________________________________                                        Zircon            55.2 g                                                      Silicon nitride   38.3 g                                                      Alumina           4.6 g                                                       Yttria            1.5 g   (represents                                                                   4% by weight                                                                  based on                                                                      ZrO.sub.2)                                          ______________________________________                                    

Composition C

    ______________________________________                                        Zircon            55.2 g                                                      Silicon nitride   38.0 g                                                      Alumina           4.6 g                                                       Yttria            2.2 g   (represents                                                                   6% by weight                                                                  based on                                                                      ZrO.sub.2)                                          ______________________________________                                    

Composition D

    ______________________________________                                        Zircon            54.8 g                                                      Silicon nitride   37.7 g                                                      Alumina           4.6 g                                                       Yttria            3.0 g   (represents                                                                   8% by weight                                                                  based on                                                                      ZrO.sub.2)                                          ______________________________________                                         Composition E

    ______________________________________                                        Zircon            54.4 g                                                      Silicon nitride   37.4 g                                                      Alumina           4.5 g                                                       Yttria            3.7 g   (represents                                                                   10% by weight                                                                 based on                                                                      ZrO.sub.2)                                          ______________________________________                                    

Composition F

    ______________________________________                                        Zircon            52.5 g                                                      Silicon nitride   36.1 g                                                      Alumina           4.4 g                                                       Yttria            7.0 g   (represents                                                                   20% by                                                                        weight based                                                                  on ZrO.sub.2)                                       ______________________________________                                    

The above compositions were fired at 1700° C. for 5 hours in a carbonelement furnace.

X-ray diffranction traces of the fired, crushed ceramics indicated thatwith no yttria addition the zirconia is monoclinic with a trace ofnitrogen stabilized cubic zirconia. The amount of tetragonal zirconiaincreases with yttria content and reaches >95% for Composition E. AnX-ray diffraction trace for composition E is given in FIG. 1. The X-raydiffraction trace was taken with copper Kα radiation. The porosity anddensity are shown in FIG. 2. With low yttria additions, insufficienttransient liquid phase is generated to give complete densification. Atmedium yttria addition the ceramic partly densifies whilst at highyttria additions the ceramic is better than 95% dense with most of thezirconia stabilized as tetragonal zirconia.

EXAMPLE 2

Zircon (55.4g), silicon nitride (38.1g) and magnesium spinel, MgAl₂ O₄.(6.4g) were thoroughly mixed together and isostatically pressed at20,000 psi. The billets were then fired for 3 hours at a temperature of1500° C. At this temperature the product was zirconia dispersed in anO'-sialon matrix. An electron micrograph of the product revealed a glassgrain boundary phase at a magnification of 5000.

EXAMPLE 3

Zirconia (36.5g), silica (17.8g), silicon nitride (37.4g), alumina(4.5g) and yttria (3.7g) were thoroughly mixed together andisostatically pressed at 20,000 psi. The billet was then fired for 5hours at 1750° C. The product was zirconia dispersed in an O'-sialonmatrix and the density was 3.58 g/cm.sup. 3.

EXAMPLE 4

Zircon (40.0g), silica (9.0g), silicon nitride (46.0g), alumina (5.5g)and yttria (2.2g) were thoroughly mixed and isostatically pressed at20,000 psi. The billet was then fired for 3 hours at 1700° C. Theproduct was fully dense and comprised 15 volume percent zirconia and 85volume percent of O'-sialon.

EXAMPLE 5

Zirconia (36.7g), silica (17.9g), silicon nitride (41.8g) and yttria(3.7g) were thoroughly mixed together and isostatically pressed at20,000 psi. The billet was then fired for 5 hours at 1700° C. Theproduct was zirconia in a silicon oxynitride matrix and the density was3.35 g/cm.sup. 3.

EXAMPLE 6

Zirconia (37.9g), silica (18.5g), silicon nitride (38.9g) and alumina(4.7g) were thoroughly mixed and isostatically pressed at 20,000 psi.The billet was then fired for 5 hours at 1700° C. The product had adensity of 3.47 g/cm.sup. 3.

FIG. 3 is an X-ray diffraction trace of this composition taken withcopper Lα radiation. This X-ray diffraction trace shows only monocliniczirconia with the tiny peak at 30° representing a trace of zirconiumoxynitride. (Yttria was ommitted from the composition because yttriastabilized ZrO₂ (tetragonal ZrO₂) would also give a peak in thisposition).

FIG. 4 is a electron micrograph of this composition taken at 1000 timesmagnification. The white phase is zirconia and the dark phase isO'-sialon. The marker on the photograph represents 10 micrometres.

COMPARATIVE EXAMPLE 7

Zircon (26.7g), silicon nitride (61.3g) and aluminium nitride (12.0g)were thoroughly mixed and isostatically pressed at 20,000 psi. Thebillet was then fired for 5 hours at 1700° C. The product was zirconiain β'-sialon and had a density of 3.46 g/cm.sup. 3. Although thematerial had densified very well without the addition of yttria, theproduct contained large amounts of zirconium oxynitride. This can beseen from FIG. 5 which is an X-ray diffraction trace of the compositiontaken with copper Lα radiation. The peak at 30° is attributable tozirconium oxynitride. Up to 25% of the zirconia is nitrogen stabilizedin this composition.

We claim:
 1. A ceramic material which consists essentially of zirconiaand O'-sialon, the zirconia being present in an amount of from 5 to 95volume percent based on the total weight of the composition.
 2. Ceramicmaterial according to claim 1 which consists essentially of a dispersionof zirconia in an O'-sialon matrix.
 3. Ceramic material according toclaim 2 which consists essentially of from 5 to 30 volume percent, ofzirconia based on the total volume of the composition.
 4. Ceramicmaterial according to claim 1, wherein the composite includes therein asolid solution of zirconia with a compound selected from the groupconsisting of yttria, ceria, lanthanum oxide, calcium oxide, magnesiumoxide and a rare earth metal oxide.
 5. Ceramic material according toclaim 3 which consists essentially of from 15 to 25 volume percent ofzirconia based on the total volume of the composition.